Ion exchange resins have been used previously in wide industrial fields of not only cleaning of water, but also medicine, food, chemical industry, and the like. In general, ion exchange resins can be classified into anion exchange resins and cation exchange resins and each has a chemical structure in which an anion exchange group or a cation exchange group is introduced in a cross-linked three-dimensional polymeric base member.
Examples of well-known anion exchange groups include primary to tertiary amino groups and an ammonium group.
In general, the anion exchange resin is produced by subjecting a copolymer of a monovinyl aromatic monomer and a cross-linkable aromatic monomer to a reaction with a haloalkylating agent, so as to introduce a haloalkyl group, and thereafter, a reaction with an amine compound.
The performance required of the anion exchange resin is different depending on the use thereof. However, it is desired in common to have an appropriate exchange capacity and an appropriate water content.
Examples of well-known cation exchange groups include a sulfonic group, a carboxyl group, and a phosphonyl group.
In general, the cation exchange resin is produced by subjecting a copolymer of a monovinyl aromatic monomer and a cross-linkable aromatic monomer to a reaction with a sulfonation agent.
Previously, the ion exchange resin including a cross-linked copolymer as a base member have had a problem in that leachables of an organic material or the like occurs during the use thereof. Such a leachable from a resin causes invitation of coloring•poisoning of a liquid to be treated through separation or refining, inhibition of desalting•odor development•reduction in amount of treatment due to pollution of a surface of the resin, an increase in water content due to decomposition of the resin, and the like. In particular, regarding ultrapure water used for silicon wafer washing, electronic component•material washing, and the like, even when the amount of leachables is very small, such a leachable may be adsorbed by a silicon wafer surface and, thereby, a product may be adversely affected. Furthermore, the silicon wafer may be etched by such a leachable, and the surface flatness may be adversely affected.
Consequently, in the use for production of ultrapure water, an ion exchange resin, in which the amount of leachables from a resin is significantly small and, in particular, the degree of leachables of the substance causing etching of the silicon wafer is reduced, has been desired.
Examples of causes of an occurrence of leachables from the resin include the presence of impurities, e.g., unpolymerized monomer components (monomers), inadequately polymerized low-molecular-weight polymer components (dimers, trimers, oligomers), separable polymer components (linear polymers, polymer fine particles), and byproducts through polymerization reaction, which remain in the production of a cross-linked copolymer, in the first place. As for remaining impurities, in the case of a styrene based resin, for example, unpolymerized monomer components include a styrene monomer, divinylbenzene, ethylvinylbenzene, and the like. Inadequately polymerized low-molecular-weight polymer components include styrene dimers, styrene trimers, styrene oligomers, and the like. Separable polymer components include linear polystyrenes, polystyrene fine particles, and the like. Byproducts through polymerization reaction include formaldehyde, benzaldehyde, and the like.
However, no means effective for preventing such impurities from remaining has been known. In order to remove such impurities, a step to wash the impurities with distilled water or the like has been previously required after the production or before the use of the ion exchange resin or a synthesized adsorbent. Therefore, an increase in cost and complication of the steps have been invited.
As for another cause of an occurrence of leachables, it is mentioned that the cross-linked copolymer is decomposed due to oxidation or the like with the passage of time so as to generate decomposition products in the use or storage thereof.
In order to prevent generation of such decomposition products, a technology of introducing a substituent to give an antioxidant capability have been proposed previously (refer to, for example, Patent Documents 1 to 3). However, the effect thereof is not satisfactory.
On the other hand, regarding the exchange capacity of the ion exchange resin, in order to minimize the frequency of replacement of the resin, previously, there has been a tendency to desire an ion exchange resin having a large exchange capacity.
In particular, regarding the ion exchange resin for ultrapure water production, a water treatment is conducted at a high flow rate. Therefore, there has been a tendency to design the structure in such a way that the water to be treated diffuses into the inside of the ion exchange resin easily and the advantage in reaction rate is offered. That is, regarding the ion exchange resin for ultrapure water production, there has been a tendency to desire a resin having not only a large exchange capacity, but also a low degree of cross-linkage and a large water content.
Furthermore, Patent Document 4 describes the possibility that a substance having a positive zeta potential relates to a proportion defective in semiconductor production to a great extent, and the substance having a positive zeta potential is derived from a strongly basic anion exchange resin. However, the evaluation in Patent Document 4 employs the degree of stains of a silicon wafer as the reference and, therefore, does not reach the evaluation at a strict level, such as the quality of flatness of the silicon wafer. Moreover, Patent Document 4 discloses a method in which the substance having a positive zeta potential is reduced by employing a mixed bed with a cation exchange resin. However, a method for reducing leachables from the anion exchange resin alone or a method for reducing substances which etch a silicon wafer is not disclosed.
That is, in the past, there is no related art in which an improvement of an ion exchange resin used for production of ultrapure water is considered from a strict viewpoint of an influence on the flatness of a silicon wafer surface.
In addition, Patent Document 5 discloses a method for suppressing leachables taking the advantage of sulfone cross-linking reaction at the stage of sulfonation. However, regarding the cation exchange resin for production of ultrapure water, in particular, production of electronic component•material washing ultrapure water, a further improvement is desired in the effect thereof.
On the other hand, independently of the ultrapure water production technology, in the case where the anion exchange resin and the cation exchange resin are used in the mixed bed, the volume of the mixed bed resin formed from the cation exchange resin and the anion exchange resin increases excessively because of an “agglomeration (clumping) phenomenon” which occurs between the cation exchange resin and the anion exchange resin, so as to cause a handling problem. As for a technology for preventing this agglomeration (clumping), a method in which an anion exchange resin is treated with a water-soluble polymer containing an anionic dissociative group has been disclosed (Patent Document 6). However, this strictly relates to the technology for preventing the agglomeration (clumping) and does not relate to an improvement technology, which suppresses an influence on the flatness of a silicon wafer surface, for production of an ion exchange resin.    Patent Document 1: European Patent Application Publication No. 1078940 A1    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2-115046    Patent Document 3: Japanese Unexamined Patent Application Publication No. 10-137736    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2003-334550    Patent Document 5: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 10-508061    Patent Document 6: Japanese Patent Application No. 9-19964
On the basis of the above-described related art, regarding the ion exchange resin (anion exchange resin, cation exchange resin) including a cross-linked copolymer, a technology for preventing remaining of impurities and generation of decomposition products and suppressing an occurrence of a leachable during the use has been desired.
Furthermore, regarding the anion exchange resin including a cross-linked copolymer, a technology for reducing leachables from a resin for the purpose of suppressing the influence on the flatness of the silicon wafer has been still more desired than ever.
The present invention has been made in consideration of the above-described problems. Accordingly, it is an object of the present invention to provide an ion exchange resin, in which remaining of impurities and generation of decomposition products are suppressed and leachables are reduced and, in particular, which is capable of realizing reduction of leachables to a level not easily deteriorating the flatness of the silicon wafer surface, a method for manufacturing the same, as well as a mixed bed resin including the ion exchange resin, and a method for manufacturing electronic component•material washing ultrapure water by using the ion exchange resin.